Big Bergs Ahoy!

An armada of ice sets sail for the new millennium

What's eating Antarctica? In March 2000, an 11,000-square-kilometer iceberg the size of Connecticut split from the Ross Ice Shelf in Antarctica. Two months later, a similar area of ice broke free from the continent's Ronne Ice Shelf. Three months after that, the Ninnis Glacier Tongue, a 1,450 sq-km slab of ice jutting into the sea, snapped off near the shoreline and cast off for warmer climes.

Last September, yet another huge chunk of ancient ice broke free from the Ross Ice Shelf. Now, satellites have detected a crack across the Antarctic ice shelf that's fed by the Pine Island Glacier. This massive fissure promises to spawn another megaberg in the next 12 to 18 months.

Scientists are asking what's behind this sudden mass exodus. Each Antarctic ice shelf typically sheds a super-size iceberg only once every few decades. Global warming might seem like the most likely explanation, but researchers have been unable to directly link warming to the rapid-fire shedding from the continent's southernmost ice shelves. In fact, scientists argue, the ice shelves were overdue to unload these huge burdens.

Antarctic icebergs are a breed apart from their Arctic brethren. Most large icebergs in the Northern Hemisphere calve from glaciers that flow into the ocean along the coasts of Alaska and Greenland. Smaller chunks come from the breakup of ice that forms on the ocean every winter.

In contrast large southern bergs, which often cover hundreds of square kilometers, periodically split off from ancient ice shelves that float atop the frigid southern seas. Nourished by the languid flow of solid kilometers-thick ice sheets overlying the continent, some of these ice shelves are hundreds of meters thick and cover shallow coastal regions the size of Texas or larger.

Although the Arctic Ocean doesn't have such perennial ice shelves these days, there are hints of their earlier presence. Scientists recently reported huge scrapes and gouges on deep undersea sediments there, which suggest that kilometer-thick ice shelves graced the Arctic seas during some previous ice ages (SN: 3/24/01, p. 181).

North or south, massive changes in the icescape normally happen slowly. There's a reason the word glacial means slow. Last year's Antarctic emigres seem to be following a new set of rules, however. They were cast away in quick succession, as if the continent's ice shelves suddenly decided to follow through collaboratively on a new-millennium resolution to shed a few billion tons of unsightly ice.

Despite appearances, it's unlikely that the coincidence of so many megabergs signals a wholesale collapse of the ice shelves, says Ted A. Scambos, a glaciologist with the National Snow and Ice Data Center in Boulder, Colo. On the whole, he notes, the edges of each of Antarctica's largest ice shelves--the Ross, the Ronne, and the Amery--fluctuate around an average position.

Before last year, all three ice shelves stretched dozens of kilometers farther from the coast than they had in decades. For instance, the western portion of the Ross Ice Shelf, discovered in 1840, reached farther from the coast in early March 2000 than ever before noted.

With such unusually large projections into the ocean, the shelves were primed to cast off big bergs. Now, the central and eastern portions of the Ross Ice Shelf have calved back to where they were in the early 1960s, says Stan Jacobs, a glaciologist at the Lamont-Doherty Earth Observatory in Palisades, N.Y.

"These calving events get a lot of attention due to the size of the icebergs, but in the long run, there's really not much net change in the position of the edge of the ice shelf," notes Jacobs.

Scambos says that it's not clear what causes an iceberg to break free from these three southern ice shelves. After all, each lies well within the region where average annual temperatures are below -5 [degrees] C and the average summer temperatures don't rise above freezing. Therefore, it's unlikely that warming temperatures caused the recent spate of megabergs, says Scambos.

Instead, he says, the factors most likely at play here are tides, ocean currents beneath the ice shelf, and so-called katabatic winds--the cold, heavy air masses that can spill off the continent with hurricane speed. All these phenomena exert tremendous forces on rifts and other weak spots that form in the ice as it flows off the continent, begins to float, and streams around and over nearby islands.

Scambos says that he and other scientists are now developing a proposal for a research project that would place Global Positioning System (GPS) equipment and weather instruments on an ice shelf where there's a known rift. A primary goal of the project would be to collect data on an iceberg-spawning crack as it grows.

In the meantime, scientists are already monitoring a newly discovered crack in the ice shelf fed by the Pine Island Glacier, a fast-flowing ice stream that's thinning more rapidly than ice on any other area in Antarctica (SN: 2/3/01, p. 70). In January, when Robert Bindschadler was looking at Landsat 7 satellite images of Antarctica, he noticed a 25-kilometer-long crack across this ice shelf.

The 400-m-wide crack hadn't been there the previous year, and analysis of recent images from other satellites shows it hadn't been there even 5 weeks earlier. After its initial growth spurt, the crack's progress slowed, and it now grows only about 13 m per day, says Bindschadler, a glaciologist at Goddard Space Flight Center.

In the next 12 to 18 months, the crevasse should completely cleave the 40-km-wide, 400-m-thick ice shelf and release an 800-square-kilometer iceberg. Although relatively meager by Antarctic standards, the berg will contain enough ice to provide every person on Earth with a 25-pound bag of ice every day for the next 5 years. "Even small things in Antarctica are huge," Bindschadler quips.

Even though global warming probably isn't to blame for last year's megabergs, Scambos says that rising temperatures have been linked to ill effects along the coast of the Antarctica's northernmost peninsula. During a summer storm in January 1995, the Larsen A Ice Shelf, which covered 2,000 square kilometers, disintegrated into a flotilla of small icebergs.

This ice shelf's next-door neighbor, Larsen B, was also pummeled by the storm. It shed a big iceberg at that time and has continued to release small bergs. The outer edges of Larsen B now are closer to the Antarctic shoreline than ever recorded in the past.

The explanation for these ice shelves' demise may be the recent rise in summertime temperatures in the area, says Scambos. He and his colleague Christina L. Hulbe, a glaciologist at the Goddard Space Flight Center in Greenbelt, Md., described this phenomenon last December at the American Geophysical Union meeting in San Francisco.

When the temperatures on the surface of the ice rise above freezing, the melting snow collects in crevasses atop the ice sheet. The pressure exerted by this meltwater deep in a crack drives it to open even further, weakening the ice shelf and making it vulnerable to winds and tides. This generates a large number of small icebergs, a process that Scambos says is much different from the one that takes place on the more southerly ice shelves.

He notes that the breakup of the Larsen B Ice Shelf has been slower during the past 2 years than previously and that satellite imagery didn't reveal any ponds of melted snow atop the ice last summer. Part of the reason for this could be La Nina conditions, a cooling of the waters in the equatorial Pacific Ocean that influences weather worldwide, Scambos says. That cooling, in turn, could have chilled air temperatures above the Antarctic ice shelves.

A return of El Nino, the warming of the equatorial Pacific, might spell the end for Larsen B because it could warm up the region and cause the ice shelf to shed even more bergs. Scambos says it's likely that the Larsen B ice shelf will continue to retreat and may even disappear within the next decade.

The icebergs that break free from the main ice shelves in Antarctica eventually make their way north and threaten the shipping lanes. The size of the bergs guarantees that they'll last a long time--one megaberg that calved in January 1992 wandered westward along the coast for 3 years before splitting in two. Then, the larger piece drifted eastward for thousands of kilometers before it finally broke up and melted in the South Atlantic Ocean in February 2000.

Before such bergs drift to tropical climes, however, they have to escape the shallow waters of coastal Antarctica. Driven by strong currents that sweep along the coast, the floating masses of ice jostle and bang their way along the ice shelf for hundreds of miles.

As the tides go out twice each day, the bergs take a long, slow slide downhill. When the tides return, the bergs slosh back into the ice shelf from whence they came, cracking off smaller bergs in the process.

Sometimes, the currents drive the megabergs aground in shallow water, where they can be stranded for years. Two pieces of a large iceberg that broke free in 1987 were soon driven aground, and they remain where they landed.

A large piece of the Connecticut-size iceberg that broke free from the Ross Ice Shelf in March 2000 jostled its way down the coast about 300 km before it was driven aground. When this piece--now merely the size of Rhode Island--became stranded near Ross Island, scientists were worried that it might break free and block the route taken by supply ships into McMurdo Sound, the site of many Antarctic research stations.

For the time being, however, the flat-bottom berg poses no threat to shipping. Even so, it may not be harmless. It's positioned near Ross Island's Cape Crozier and poses a direct and unusual threat to some of the area's wildlife (see Icy obstacle blocks to route to rookery).

Recently, scientists visited this iceberg and installed GPS equipment and weather stations. If the berg ever floats again and moves into warmer waters, these instruments will help monitor its eventual breakup, says Douglas R. MacAyeal, a glaciologist at the University of Chicago.

It wouldn't be hard for people on a ship to detect and then steer around an iceberg that spans the horizon and towers higher than a 12-story building above the waves. However, when these frozen leviathans spawn legions of smaller bergs--similar in size to the pipsqueak ice cube that sank the Titanic--they give sea traffic reason to fear.

RELATED ARTICLE: Icy obstacle blocks route to rookery

While a grounded iceberg near Ross Island's Cape Crozier in Antarctica poses no problem for shipping, it does threaten many thousands of penguins that breed there, says David Ainley, a marine ecologist with H.T. Harvey & Associates, an ecological consulting firm in San Jose, Calif.

Adelie penguins spend most of their lives at sea in feeding grounds that extend out about 70 kilometers from their Cape Crozier rookery, the largest in the area. But once every 2 years, the rookery's 120,000 or so pairs of Adelies come ashore to breed and lay their eggs.

Shortly before the chicks are grown, the adults return to the sea to feed on krill and fish. The chicks find their way to sea on their own soon afterwards.

Last October, while the iceberg was still on the move, the adult penguins swam to their feeding grounds through a 10-km-wide avenue of ocean. When the flat-bottom berg became stuck, that lane had narrowed to a 1-km alley.

Furthermore, the berg might be there a while: Because it's aground on the sea floor, the bottom of the iceberg won't be exposed to relatively warm ocean water that might eventually melt it free.

When it comes time for the penguins to return to their rookery, they'll find a sheer cliff of ice that stretches from the ocean floor to the sky--a wall they can't swim under or waddle over.

Adelie penguins are faithful to their colonies, even to the point of coming back to the same nest, says Ainley. When scientists studied the navigational abilities of Adelie penguins in the late 1960s, birds from the Cape Crozier rookery were relocated more than 1,000 km away.

It took the deported Addies at least a year to return, Ainley notes, but return they did. This homing instinct, which normally brings the birds back to a familiar and safe breeding colony, may turn out to be their Achilles heel.

"It will be interesting to see if this iceberg causes the penguins to change their behavior and emigrate to other colonies," says Ainley.

A small but rapidly-growing colony of about 1,600 emperor penguins, which lay their eggs in the same rookery, have been feeding at sea. Like the Addies, they'll soon come back to face the daunting wall of ice, Ainley says. --S.P